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The Commons Collections LRUMap.java source code
/*
* Licensed to the Apache Software Foundation (ASF) under one or more
* contributor license agreements. See the NOTICE file distributed with
* this work for additional information regarding copyright ownership.
* The ASF licenses this file to You under the Apache License, Version 2.0
* (the "License"); you may not use this file except in compliance with
* the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.apache.commons.collections.map;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;
import java.util.Map;
import org.apache.commons.collections.BoundedMap;
/**
* A <code>Map implementation with a fixed maximum size which removes
* the least recently used entry if an entry is added when full.
* <p>
* The least recently used algorithm works on the get and put operations only.
* Iteration of any kind, including setting the value by iteration, does not
* change the order. Queries such as containsKey and containsValue or access
* via views also do not change the order.
* <p>
* The map implements <code>OrderedMap and entries may be queried using
* the bidirectional <code>OrderedMapIterator. The order returned is
* least recently used to most recently used. Iterators from map views can
* also be cast to <code>OrderedIterator if required.
* <p>
* All the available iterators can be reset back to the start by casting to
* <code>ResettableIterator and calling reset().
* <p>
* <strong>Note that LRUMap is not synchronized and is not thread-safe.
* If you wish to use this map from multiple threads concurrently, you must use
* appropriate synchronization. The simplest approach is to wrap this map
* using {@link java.util.Collections#synchronizedMap(Map)}. This class may throw
* <code>NullPointerException's when accessed by concurrent threads.
*
* @since Commons Collections 3.0 (previously in main package v1.0)
* @version $Revision: 646777 $ $Date: 2008-04-10 13:33:15 +0100 (Thu, 10 Apr 2008) $
*
* @author James Strachan
* @author Morgan Delagrange
* @author Stephen Colebourne
* @author Mike Pettypiece
* @author Mario Ivankovits
*/
public class LRUMap
extends AbstractLinkedMap implements BoundedMap, Serializable, Cloneable {
/** Serialisation version */
private static final long serialVersionUID = -612114643488955218L;
/** Default maximum size */
protected static final int DEFAULT_MAX_SIZE = 100;
/** Maximum size */
private transient int maxSize;
/** Scan behaviour */
private boolean scanUntilRemovable;
/**
* Constructs a new empty map with a maximum size of 100.
*/
public LRUMap() {
this(DEFAULT_MAX_SIZE, DEFAULT_LOAD_FACTOR, false);
}
/**
* Constructs a new, empty map with the specified maximum size.
*
* @param maxSize the maximum size of the map
* @throws IllegalArgumentException if the maximum size is less than one
*/
public LRUMap(int maxSize) {
this(maxSize, DEFAULT_LOAD_FACTOR);
}
/**
* Constructs a new, empty map with the specified maximum size.
*
* @param maxSize the maximum size of the map
* @param scanUntilRemovable scan until a removeable entry is found, default false
* @throws IllegalArgumentException if the maximum size is less than one
* @since Commons Collections 3.1
*/
public LRUMap(int maxSize, boolean scanUntilRemovable) {
this(maxSize, DEFAULT_LOAD_FACTOR, scanUntilRemovable);
}
/**
* Constructs a new, empty map with the specified initial capacity and
* load factor.
*
* @param maxSize the maximum size of the map, -1 for no limit,
* @param loadFactor the load factor
* @throws IllegalArgumentException if the maximum size is less than one
* @throws IllegalArgumentException if the load factor is less than zero
*/
public LRUMap(int maxSize, float loadFactor) {
this(maxSize, loadFactor, false);
}
/**
* Constructs a new, empty map with the specified initial capacity and
* load factor.
*
* @param maxSize the maximum size of the map, -1 for no limit,
* @param loadFactor the load factor
* @param scanUntilRemovable scan until a removeable entry is found, default false
* @throws IllegalArgumentException if the maximum size is less than one
* @throws IllegalArgumentException if the load factor is less than zero
* @since Commons Collections 3.1
*/
public LRUMap(int maxSize, float loadFactor, boolean scanUntilRemovable) {
super((maxSize < 1 ? DEFAULT_CAPACITY : maxSize), loadFactor);
if (maxSize < 1) {
throw new IllegalArgumentException("LRUMap max size must be greater than 0");
}
this.maxSize = maxSize;
this.scanUntilRemovable = scanUntilRemovable;
}
/**
* Constructor copying elements from another map.
* <p>
* The maximum size is set from the map's size.
*
* @param map the map to copy
* @throws NullPointerException if the map is null
* @throws IllegalArgumentException if the map is empty
*/
public LRUMap(Map map) {
this(map, false);
}
/**
* Constructor copying elements from another map.
* <p/>
* The maximum size is set from the map's size.
*
* @param map the map to copy
* @param scanUntilRemovable scan until a removeable entry is found, default false
* @throws NullPointerException if the map is null
* @throws IllegalArgumentException if the map is empty
* @since Commons Collections 3.1
*/
public LRUMap(Map map, boolean scanUntilRemovable) {
this(map.size(), DEFAULT_LOAD_FACTOR, scanUntilRemovable);
putAll(map);
}
//-----------------------------------------------------------------------
/**
* Gets the value mapped to the key specified.
* <p>
* This operation changes the position of the key in the map to the
* most recently used position (first).
*
* @param key the key
* @return the mapped value, null if no match
*/
public Object get(Object key) {
LinkEntry entry = (LinkEntry) getEntry(key);
if (entry == null) {
return null;
}
moveToMRU(entry);
return entry.getValue();
}
//-----------------------------------------------------------------------
/**
* Moves an entry to the MRU position at the end of the list.
* <p>
* This implementation moves the updated entry to the end of the list.
*
* @param entry the entry to update
*/
protected void moveToMRU(LinkEntry entry) {
if (entry.after != header) {
modCount++;
// remove
entry.before.after = entry.after;
entry.after.before = entry.before;
// add first
entry.after = header;
entry.before = header.before;
header.before.after = entry;
header.before = entry;
} else if (entry == header) {
throw new IllegalStateException("Can't move header to MRU" +
" (please report this to commons-dev@jakarta.apache.org)");
}
}
/**
* Updates an existing key-value mapping.
* <p>
* This implementation moves the updated entry to the top of the list
* using {@link #moveToMRU(AbstractLinkedMap.LinkEntry)}.
*
* @param entry the entry to update
* @param newValue the new value to store
*/
protected void updateEntry(HashEntry entry, Object newValue) {
moveToMRU((LinkEntry) entry); // handles modCount
entry.setValue(newValue);
}
/**
* Adds a new key-value mapping into this map.
* <p>
* This implementation checks the LRU size and determines whether to
* discard an entry or not using {@link #removeLRU(AbstractLinkedMap.LinkEntry)}.
* <p>
* From Commons Collections 3.1 this method uses {@link #isFull()} rather
* than accessing <code>size and maxSize directly.
* It also handles the scanUntilRemovable functionality.
*
* @param hashIndex the index into the data array to store at
* @param hashCode the hash code of the key to add
* @param key the key to add
* @param value the value to add
*/
protected void addMapping(int hashIndex, int hashCode, Object key, Object value) {
if (isFull()) {
LinkEntry reuse = header.after;
boolean removeLRUEntry = false;
if (scanUntilRemovable) {
while (reuse != header && reuse != null) {
if (removeLRU(reuse)) {
removeLRUEntry = true;
break;
}
reuse = reuse.after;
}
if (reuse == null) {
throw new IllegalStateException(
"Entry.after=null, header.after" + header.after + " header.before" + header.before +
" key=" + key + " value=" + value + " size=" + size + " maxSize=" + maxSize +
" Please check that your keys are immutable, and that you have used synchronization properly." +
" If so, then please report this to commons-dev@jakarta.apache.org as a bug.");
}
} else {
removeLRUEntry = removeLRU(reuse);
}
if (removeLRUEntry) {
if (reuse == null) {
throw new IllegalStateException(
"reuse=null, header.after=" + header.after + " header.before" + header.before +
" key=" + key + " value=" + value + " size=" + size + " maxSize=" + maxSize +
" Please check that your keys are immutable, and that you have used synchronization properly." +
" If so, then please report this to commons-dev@jakarta.apache.org as a bug.");
}
reuseMapping(reuse, hashIndex, hashCode, key, value);
} else {
super.addMapping(hashIndex, hashCode, key, value);
}
} else {
super.addMapping(hashIndex, hashCode, key, value);
}
}
/**
* Reuses an entry by removing it and moving it to a new place in the map.
* <p>
* This method uses {@link #removeEntry}, {@link #reuseEntry} and {@link #addEntry}.
*
* @param entry the entry to reuse
* @param hashIndex the index into the data array to store at
* @param hashCode the hash code of the key to add
* @param key the key to add
* @param value the value to add
*/
protected void reuseMapping(LinkEntry entry, int hashIndex, int hashCode, Object key, Object value) {
// find the entry before the entry specified in the hash table
// remember that the parameters (except the first) refer to the new entry,
// not the old one
try {
int removeIndex = hashIndex(entry.hashCode, data.length);
HashEntry[] tmp = data; // may protect against some sync issues
HashEntry loop = tmp[removeIndex];
HashEntry previous = null;
while (loop != entry && loop != null) {
previous = loop;
loop = loop.next;
}
if (loop == null) {
throw new IllegalStateException(
"Entry.next=null, data[removeIndex]=" + data[removeIndex] + " previous=" + previous +
" key=" + key + " value=" + value + " size=" + size + " maxSize=" + maxSize +
" Please check that your keys are immutable, and that you have used synchronization properly." +
" If so, then please report this to commons-dev@jakarta.apache.org as a bug.");
}
// reuse the entry
modCount++;
removeEntry(entry, removeIndex, previous);
reuseEntry(entry, hashIndex, hashCode, key, value);
addEntry(entry, hashIndex);
} catch (NullPointerException ex) {
throw new IllegalStateException(
"NPE, entry=" + entry + " entryIsHeader=" + (entry==header) +
" key=" + key + " value=" + value + " size=" + size + " maxSize=" + maxSize +
" Please check that your keys are immutable, and that you have used synchronization properly." +
" If so, then please report this to commons-dev@jakarta.apache.org as a bug.");
}
}
/**
* Subclass method to control removal of the least recently used entry from the map.
* <p>
* This method exists for subclasses to override. A subclass may wish to
* provide cleanup of resources when an entry is removed. For example:
* <pre>
* protected boolean removeLRU(LinkEntry entry) {
* releaseResources(entry.getValue()); // release resources held by entry
* return true; // actually delete entry
* }
* </pre>
* <p>
* Alternatively, a subclass may choose to not remove the entry or selectively
* keep certain LRU entries. For example:
* <pre>
* protected boolean removeLRU(LinkEntry entry) {
* if (entry.getKey().toString().startsWith("System.")) {
* return false; // entry not removed from LRUMap
* } else {
* return true; // actually delete entry
* }
* }
* </pre>
* The effect of returning false is dependent on the scanUntilRemovable flag.
* If the flag is true, the next LRU entry will be passed to this method and so on
* until one returns false and is removed, or every entry in the map has been passed.
* If the scanUntilRemovable flag is false, the map will exceed the maximum size.
* <p>
* NOTE: Commons Collections 3.0 passed the wrong entry to this method.
* This is fixed in version 3.1 onwards.
*
* @param entry the entry to be removed
*/
protected boolean removeLRU(LinkEntry entry) {
return true;
}
//-----------------------------------------------------------------------
/**
* Returns true if this map is full and no new mappings can be added.
*
* @return <code>true if the map is full
*/
public boolean isFull() {
return (size >= maxSize);
}
/**
* Gets the maximum size of the map (the bound).
*
* @return the maximum number of elements the map can hold
*/
public int maxSize() {
return maxSize;
}
/**
* Whether this LRUMap will scan until a removable entry is found when the
* map is full.
*
* @return true if this map scans
* @since Commons Collections 3.1
*/
public boolean isScanUntilRemovable() {
return scanUntilRemovable;
}
//-----------------------------------------------------------------------
/**
* Clones the map without cloning the keys or values.
*
* @return a shallow clone
*/
public Object clone() {
return super.clone();
}
/**
* Write the map out using a custom routine.
*/
private void writeObject(ObjectOutputStream out) throws IOException {
out.defaultWriteObject();
doWriteObject(out);
}
/**
* Read the map in using a custom routine.
*/
private void readObject(ObjectInputStream in) throws IOException, ClassNotFoundException {
in.defaultReadObject();
doReadObject(in);
}
/**
* Writes the data necessary for <code>put() to work in deserialization.
*/
protected void doWriteObject(ObjectOutputStream out) throws IOException {
out.writeInt(maxSize);
super.doWriteObject(out);
}
/**
* Reads the data necessary for <code>put() to work in the superclass.
*/
protected void doReadObject(ObjectInputStream in) throws IOException, ClassNotFoundException {
maxSize = in.readInt();
super.doReadObject(in);
}
}
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